B cells are the central source of long-term humoral immune responses to viral pathogens, but also serve critical regulatory functions during adaptive CD4+ T cell responses. It is unknown whether B cells contribute significantly to viral immune responses beyond antibody production or whether their manipulation can hasten or enhance immune responses. Important for emerging infections and biodefense, we have recently shown that B cells are essential for optimal CD4+ T cell priming during bacteria challenge. By contrast, monoclonal antibody (mAb)-induced B cell depletion augments Th1-type cellular immune responses in other models. This unexpected observation is explained by the identification of a potent regulatory B cell subset that dramatically attenuates Th1 immune responses and autoimmunity. We have labeled this phenotypically unique B cell subset that also secretes IL-10 as B10 cells, which represent ~1% of total spleen B cells in young mice, and <1% of circulating human B cells. B10 cell numbers within tissues increase significantly in mice with autoimmunity and age. Thus, humoral and CD4+ T cell immune responses are balanced by both positive and negative B cell regulation. We have also identified a critical signaling pathway that is required For B10 cell survival in vivo. MAbs that inhibit this B cell-restricted survival signal induce rapid and semiselective B10 cell depletion in vivo, which has an adjuvant-like effect that enhances humoral antibody responses to T cell-dependent model antigens and Thl-type CD4+ T cell immune responses. Thus, B10 cells regulate both humoral and Th1 immune responses. We have also developed a humanized mAb to the same survival target and generated transgenic mice expressing the human survival receptor that will facilitate preclinical translation of these basic studies into human studies. The ability to manipulate B cell contributions to humoral and cell-mediated immunity by mAb treatment offers a new strategy for accelerating mmune responses during acute pathogen challenge. The focus of our proposed studies is to identify the extent that B cells and the B10 subset modulate humoral and cellular immune responses to alphaviruses, and to determine how B cells can be manipulated for therapeutic benefit and vaccine development.